I know some of you have already seen this, but thought I'd toss it in here for anyone browsing for an engine. When I last looked at Paradox about a year ago, it was still completely under development. Now, it's still in alpha, but open source and actively being developed on. The team also provides a binary of a stable release which you can use royalty-free. The open source code is royalty-free of course, but it follows a different set of licenses (all with varying restrictions).

The editor itself is decent, albeit I was a bit confused by it (I'm not really an asset designer). The code is very nicely architected with an event driven design. It also comes with Visual Studio integration (you can get a free Community Edition of Visual Studio now, if you didn't know), which is quite handy.

As of now, documentation is still lacking, and there's no short-term, or even medium-term plans for Networking capabilities. You'd have to roll your own.

To my knowledge, Paradox is one of a handful of game engines built on top of MonoGame and uses Xamarin for cross-platform deployment. I have never actually checked it out, though. It does look impressive.